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Solid State Communications 150 (2010) 1840–1844

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Solid State Communications

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Structural and magnetic properties of Si1−xCoxC

Bo Song a,b,c,e,∗, Xiaolong Chen c, Jiecai Han e, Jikang Jian d, Hui Li c, Huiqiang Bao c, Kaixing Zhu c,Hongbo Zuo e, Xinghong Zhang e, Wanyan Wang c, Songhe Meng ea Research Station on Material Science and Engineering for Postdoctoral Fellows, Harbin Institute of Technology, Harbin 150080, Chinab Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, Chinac Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, Chinad Department of Physics, Xinjiang University, Urumchi 830046, Chinae Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, China

a r t i c l e i n f o

Article history:Received 30 September 2009Received in revised form5 May 2010Accepted 26 June 2010by B.-F. ZhuAvailable online 3 July 2010

Keywords:A. Magnetically ordered materialsA. SemiconductorsD. Magnetization

a b s t r a c t

We reported the structural and magnetic properties characterizations of Si1−xCoxC (0.0016 ≤ x ≤0.0347). X-ray diffraction and transmission electron microscopy analysis showed the highly-crystallinesingle-phase Si1−xCoxC (0.0016 ≤ x ≤ 0.0347) samples were obtained. The measurement of magneticproperties showed that the typical room temperature ferromagnetic orders were established. It wasspeculated that the complex effects were responsible for the long-rangemagnetic properties of Si1−xCoxC(0.0016 ≤ x ≤ 0.0347).

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Being inspired by utilizing the charge and spin properties ofelectrons simultaneously, dilutedmagnetic semiconductors (DMS)have received much attention from the science community since1996 [1]. As one of the most important wide-gap semiconductors,SiC [3.03 eV for 6H-SiC at room temperature (RT)] was consideredas a promising DMS matrix material due to its outstanding intrin-sic characters such as high thermal conductivity [4.9W/cm K at RTfor 6H-SiC], high breakdown field (2.4 × 106 V/cm at 100 V for6H-SiC), and high saturation velocity (2.0 × 107 cm/s for 6H-SiC,E at 2 × 105 V/cm) etc. [2,3]. Similarly, there also exist two chal-lenges in TM-doped SiC (TM = Mn, Fe, Co, Ni, etc.) as in otherDMS systems to make themselves the potential candidates ofDMSs: (1) the realization of Curie temperature (Tc) above or aroundRT; (2) whether the obtained materials are indeed a true solid so-lution of Si1−xTMxC, or remain as a SiC matrix with embeddedTM clusters, precipitates or secondary phases that are responsi-ble for the observed long-range magnetic order. Especially thelatter one, the efforts to investigate the natural ferromagnetism

∗ Corresponding address: P.O. Box 3010, 2 Yikuang Street, Nangang Dist., 150080Harbin, China. Tel.: +86 15845006406; fax: +86 10 82649646.E-mail address: [email protected] (B. Song).

(FM) origin in DMS has become a hot issue in condensed matterphysics [4–9]. For SiC-based DMSs, there exists another intractablechallenge: (3) the coexistence of SiC polytypes in high concentra-tion TM-doped SiC, including 2H-, 4H-, 6H-, 3C-, 15R-, etc. crys-tal structures observed by Song et al. [10]. SiC-based DMSs thusrequired more efforts than traditional GaN-, ZnO- or GaAs-basedDMSs to resolve challenge (3) [10,11]. Unfortunately, until now,there were only few studies on the synthesis and magnetic inves-tigations on TM-doped SiC-based DMSs. Especially, challenge (3)for high concentration TM-doped SiC has not attracted enough at-tention. Take cobalt for instance, Co-doped SiC was predicted toexhibit long-range magnetic order in 2001 by Gubanov et al. [12]while the corresponding experimental investigations on Si1−xCoxCare still lacking now.More recently, Chen et al. reported a very attractive result that

Al could play a special role in stabilizing SiC crystal structuresas 4H-, while the natural mechanism still remains unclear [13].Thus, it is easy to propose a codoped strategy as in other DMSs toresolve challenge (3); that using Al with another TM as the doubledoping elements, such as (Al, Fe), (Al, Mn), (Al, Co) or (Al, Ni) etc.to approach the single-phase SiC-based DMSs. Then, to investigatewhether (TM, Al)-codoped simple-phase SiC could also resolve thechallenges of both (1) and (2) via the optimization of experimentalconditions. Before the properties of (Al, TM)-codoped SiCwerewell

0038-1098/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2010.06.044

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Fig. 1. X-ray diffraction pattern of C1, C2 , C3 and C4 . Vertical bars at the bottom are expected Bragg positions for 6H-SiC (ICDD-PDF: 29-1128). Inset: the evolution of cellparameters a and c of Si1−xCoxC as a function of Co content x. The lines are guided to the eyes.

Fig. 2. XPS spectra for C1, C2 , C3 and C4 .

investigated, it is important to know the corresponding magneticproperties of single TM elements doped SiC. In this study, wesynthesized a series of Co-doped SiC with different Co contents.By X-ray diffraction (XRD), high-resolution transmission electronmicroscopy (HRTEM) and X-ray photoelectron (XPS) spectra, thehighly-crystalline qualities of Co-doped SiC sampleswere obtainedand no impurities or secondary phases were detected. Magneticmeasurement showed the RT FM order was well established in as-prepared Co-doped SiC samples.

2. Experiments

Sample preparation: High-purity, silicon (99.999%), carbon(99.9995%) and cobalt (99.999%) powder, supplied by Alfa Ae-sar, were selected as the starting materials. They were mixed andloaded into a graphite crucible. Then the crucible was transferredinto an induction furnace. The furnacewas evacuated (<10−4 Torr)and filled with high-purity argon (99.999%) of 0.07 MPa. Induc-tion furnace was heated to 1300 °C and held for 1 h, followed by

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Fig. 3. HRTEM images of C2 and C3 . Inset is the corresponding FFT image.

increasing the temperature to 1800 °C at a rate of 60 °C/h, and,lastly, increasing the temperature to 2200 °C at a speed of 175 °C/h,maintained for 5 h. The power was shut off and the furnace wascooled down to RT naturally.Characterization: The obtained powders were treated with asolution of concentrated HF and HNO3 with a molar ratio of 3:1at 150 °C for 2 h, and then rinsed using deionized (DI) water.Phase identifications were performed on X-ray diffractometer(Philips X’PERT MPD) with a Cu Kα radiation (λ = 1.5406 Å)operated at 40 kV and 40 mA. Bulk composition analysis wasconducted by using inductively coupled plasma-atomic emissionspectrometry (ICP-AES). A transmission electronmicroscope (TEM,JEM-2010) was used to investigate the detailed microstructures.X-ray photoelectron spectroscopy (XPS) spectra were recordedon a VG MKII spectrometer at RT using an Al Kα X-ray source(hν = 1486.6 eV). The samples were wrapped by capsule formagnetic measurement. The field-cooling (FC) and zero-field-cooling (ZFC) temperature dependence of magnetization wereperformed with a superconducting quantum interference device(SQUID, MPMS-7, Quantum Design) in the range from 5 to 300 K

and the magnetization hysteresis curves were measured at 50 Kand 300 K, respectively, in the field up to 5 kOe. The contributionfrom capsule was subtracted from the original data.

3. Results and discussion

Fig. 1 shows the XRD pattern of Si1−xCoxC with x = 0.0016,0.0076, 0.0323 and 0.0347, determined by ICP-AES, definedas C1, C2, C3, and C4, respectively. It is evident that only thepeaks corresponding to 6H-SiC (ICDD-PDF: 29-1128) phase weredetected, and trace impurity phase of carbon (ICDD-PDF: 26-1079)exists within the sensitivity of X-ray diffraction diffractometer(Philips X’PERT MPD). Compared with Mn-doped SiC [8], thepolytype phenomena in Co-doped SiC could be neglected probablydue to the trace doping content of Co elements with the formationof single-phase Co-doped SiC. XRD data were further analyzed bythe Rietveld technique using FULLPROF program [14]. The inset ofFig. 1 shows the evolution of both cell parameters a and c withthe increase of x. Both a and c decrease with the increase of xsuggesting that Co may replace Si atoms since the ionic radius ofCo2+ (0.72 Å) or Co3+ (0.63 Å) is smaller than the covalent radiusof Si atom (1.11 Å) and almost equal with the covalent radius ofC atom (0.77 Å). The evolution reveals a possible homogeneousstatistical substitution of Co2+ (Co3+) for Si in the solid solutionof Si1−xCoxC. Sha et al. [15] reported the presence of CoSi2 andCoSi phases in Co-doped SiC films which were not detected inthis study as shown in Fig. 1, indicating the solubility limit ofCo in SiC should exceed x = 0.03389 (C4). In fact, Co elementshave two possible positions in SiC crystal lattice: (1) occupy thepositions of Si sites (Corepla); (2) locate at the interstitial sites(Cointer), that is, Cototal = Corepla + Cointer. Generally, Corepla isextremely larger than Cointer and for simplicity, Cointer was ignoredfor Cototal determination by ICP-AES, but was used to denote Coreplain the following discussions.To validate the actual valence electron states of Co ions in SiC,

Fig. 2 presents XPS spectra of Si1−xCoxC (0.0016 ≤ x ≤ 0.0347).The charge-shifted spectrawere corrected using the adventitious C1s photoelectron signal at 285 eV. Because of the trace Co contents,typical XPS signal could not be identified in C1 and C2 in whichonly the noise signals from XPS were recorded in spectra. Withthe increase of x, the signal to noise gradually became larger andbinding energy (BE) peak at 781.24 and 795.9 eV, as indicated byarrows for C3 and C4 can be ascribed to Co 2p3/2 and Co 2p1/2,respectively. Other two BE peaks at higher core level are thecorresponding satellites peaks of Co 2p3/2 and 2p1/2, respectively.Compared to the values given by Ding et al. [16], the Co elementseems to behave as 3+ valence state, predominantly substitute forsilicon atoms and the energy difference betweenCo2p3/2 and2p1/2peak is∼14.66 eV, which is slightly lesser than the reported valuesfor Co3+ (15.2–15.3 eV) in oxides [17]. It is well known that theBE peaks for Co3+ are close to the values reported for Co2+, butthe fingerprints (shake up peak located at 780.4–785.5 eV for Co2p3/2) for Co2+ was not found in this study [17]. So, the possibilityof the formation of CoSi2 can be eliminated, which also can befurther demonstrated by XRD as shown in Fig. 1. Furthermore,the presence of Co4+ can be eliminated due to its characteristicCo 2p3/2 peak located at ∼786 eV which was not detected in thisstudy. Similarly, the existence of Co metallic clusters could alsobe excluded since its Co 2p3/2 position is ∼2 eV smaller thanthat of Co3+. The above results strongly suggest that Co atomswere successfully incorporated into SiC lattice without formationof any detectable impurity phases. High-resolutionmicrostructureanalysis is an intuitionistic and effective means to estimate thecrystalline qualities of as-prepared Si1−xCoxC samples. Fig. 3 showsthe representativeHRTEM images of randomly selected particles inC2 and C3, revealing the absence of any secondary phases includingintergrowths and nanoclusters. A close inspection of Fast Fourier

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Fig. 4. (a) Temperature dependence of the FC and ZFC curves at 1 kOe for C1, C2 , C3 and C4 . (b) The magnetization loops for C1, C2, C3 and C4 at 50 K. (c) The magnetizationloops for C1, C2, C3 and C4 at 300 K. (d) Evolution ofMr as a function of Co content at 50 K and 300 K, respectively. The lines are guided to the eyes.

Transform images (FFT, inset in Fig. 3) clearly shows the single-crystal nature of the obtained Si1−xCoxC. For C3 and C4, energy-dispersive X-ray spectroscopy (EDS, not shown here) carefullytaken on different particles and at different spots of one particleconfirmed the uniform distribution of Co in different particles aswell as within one particle.The temperature dependence of magnetic (M–T ) measurement

is very sensitive to magnetic impurity phases embedded in theas-prepared samples. Fig. 4(a) shows the ZFC and FC curves ofSi1−xCoxC (0.0016 ≤ x ≤ 0.0347) samples. The smooth andfeatureless M–T curves indicated the absence of any secondarymagnetic phase and confirmed the single-phase nature of theproducts, in agreement with previous XRD and HRTEM results. Incontrast with Mn-doped SiC [11], no typical FM transitions areobserved in the temperature range from 5 to 300 K in this study.According to previous investigation results on Ga1−xMnxAs, theconcavity in M–T curve can be considered as a signature of smallfree carrier concentration (nc) and small carrier mean free path(r0) [18]. Here, with the increase of x, the concave behavior ofmagnetizations for Si1−xCoxC has no obvious changes, indicatesthat the localized nature of the carriers serves as the spin couplingforce, rather than the free carriers, in determining theM–T curvescharacteristic for Si1−xCoxC. Fig. 4(b) and (c) show the comparisonof the well-defined hysteresis loops of Si1−xCoxC (0.0016 ≤ x ≤0.0347) measured in the range of 0 to±6000 Oe at 50 K and 300 K,respectively, indicating the robust FM order dominated Si1−xCoxC.One can see clearly that with the increase of x, the FM orderingwas enhanced and the remanent magnetization (Mr ) exhibitedincrements as shown in Fig. 4(d).

For the natural FM origin in Co-doped SiC, the possible impurityeffects could be excluded since the starting materials of siliconand graphite powder do not exhibit long-range magnetic orderfeatures. The possible magnetic effect from the contribution ofmetallic cobalt can also be eliminated according to the XPS results.Furthermore, the absence of any secondary magnetic phases hasruled out the possibility that FM order is due to extrinsic origin.Even if some tiny parasitic phase was not detected, the FM signalcannot be ascribed to the impurity magnetic phases since theredo not exist any ternary compounds in Co–Si–C system and forCo–Si or Co–C binary compounds, only CoSi and CoSi2 couldexist at 1500 °C and nothing could sustain the extreme reactionenvironment at 2200 °C as reported by Guo et al. [19] That is tosay, the FMorder in this studymust be intrinsic. Since only trace Coatoms were introduced into the SiC matrix, especially for the caseof C1 and C2, the large Co–Co distance make the direct interactionor Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interactionunable to occur due to the low nc . Recently, the bound magneticpolarons (BMPs) model proposed by Coey et al. [20] was thoughtto be responsible for the high resistivity samples with the nc largerthan 1020 cm−3 [21]. For the case of C1, the nc was calculatedto be for ∼1.52 × 1020 cm−3 by a simple means as reported inRef. [10], which is larger than the critical nc of BMPs model. Thus,the magnetic contribution from BMPs model for Si1−xCoxC shouldalso be taken into account.As reported by Song et al. and Li et al., very low doping

contents of TM in Fe [10], Mn-doped SiC [11], Mn-doped GaN [22]and Ni-doped [23], Mn-doped [24] AlN-based DMSs etc. couldnot induce robust intrinsic FM order without the assistance of

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defects. Similarly, the defects in SiC produced by Co doping are alsobelieved to play a significant role for the observed FM order. Thedefects in Si1−xCoxC exhibit in many forms such as dislocations,vacancies (VSi), and interstitial defects (Sii, Ci) etc. Recent studiesshowed that many types of defects could establish the long-range magnetic order [4,5,24–28]. In addition, as-prepared Co-doped SiC is a p-type semiconductor and the potential resonancesof Co2+ + h+acceptor ↔ Co3+ indicate the possible coexistenceof Co2+ and Co3+ ions, although the latter was not detectedunder the XPS resolution. Thus, the effect of superexchangeinteraction between Co2+ and Co3+ ions on FM order shouldalso be considered. It is speculated that the mixture mechanismsincluding the BMPs model, effect of defects, antiferromagnetic(AFM) interaction should jointly take responsible for the observedFM behavior in Si1−xCoxC (0.0016 ≤ x ≤ 0.0347). It also shouldbe noted that more precise characterization means such as X-rayabsorption near edge structure (XANES) or extended X-rayabsorption fine structure (EXAFS) are still required to further probethe charge states and local structural environment of Co ionswhichwill benefit us to understand the magnetic interaction mechanismand the natural FM origin in Co-doped SiC.In conclusion, Co-doped SiC samples were synthesized by

solid-state reaction. Structural analysis showed that the highly-crystalline single-phase Co-doped 6H-SiC were obtained whilethe lattice constants a and c both decreased with the increase ofCo content x. The magnetic property measurements showed thatRT FM was established through doping Co element. This studyprovides further useful information to investigate the FM originin SiC-based DMSs, and further research interests on magneticproperties of the (Al, TM) codoped-SiC will be accelerated.

Acknowledgements

This work is supported financially by the National NaturalScience Foundation of China (grant No. 50902037, 50862008),Ludo Frevel Crystallography Scholarship Award for Dr. Bo Song(The International Centre for Diffraction Data, ICDD, USA), ChinaPostdoctoral Science Foundation funded project (20090451008),Development Program for Outstanding Young Teachers in HarbinInstitute of Technology (HIT) HITQNJS. 2009. 065., Research

foundation for Young Innovation Talents of Science and Technol-ogy in Harbin (2010RFQXG018) and the Chinese Academy of Sci-ences.

References

[1] H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, Y. Iye, Appl.Phys. Lett. 69 (1996) 363.

[2] S.B. Ma, Y.P. Sun, B.C. Zhao, P. Tong, X.B. Zhu, W.H. Song, Physica B 394 (2007)122.

[3] N. Theodoropoulou, A.F. Hebard, S.N.G. Chu, M.E. Overberg, C.R. Abernathy,S.J. Pearton, R.G. Wilson, J.M. Zavada, Y.D. Park, J. Vac. Sci. Technol. A 20 (2002)579.

[4] C. Madhu, A. Sundaresan, C.N.R. Rao, Phys. Rev. B 77 (2008) 201306(R).[5] Pratibha Dev, Yu Xue, Peihong Zhang, Phys. Rev. Lett. 100 (2008) 117204.[6] F. Pan, C. Song, X.J. Liu, Y.C. Yang, F. Zeng, Mater. Sci. Eng. R 62 (2008) 1.[7] X.J. Liu, X.Y. Zhu, C. Song, F. Zeng, F. Pan, J. Phys. D: Appl. Phys. 42 (2009)035004.

[8] X.J. Liu, C. Song, F. Zeng, F. Pan, B. He,W.S. Yan, J. Appl. Phys. 103 (2008) 093911.[9] C. Song, S.N. Pan, X.J. Liu, X.W. Li, F. Zeng, W.S. Yan, B. He, F. Pan, J. Phys.:Condens. Matter 19 (2007) 176229.

[10] B. Song, J.K. Jian, H. Li, M. Lei, H.Q. Bao, X.L. Chen, G.Wang, Physica B 403 (2008)2897.

[11] B. Song, H.Q. Bao, H. Li, M. Lei, J.K. Jian, J.C. Han, X.H. Zhang, S.H. Meng,W.Y. Wang, X.L. Chen, Appl. Phys. Lett. 94 (2009) 102508.

[12] V.A. Gubanov, C. Boekema, C.Y. Fong, Appl. Phys. Lett. 78 (2001) 216.[13] B. Song, H.Q. Bao, H. Li, M. Lei, T.H. Peng, J.K. Jian, J. Liu, W.Y. Wang, W.J. Wang,

X.L. Chen, J. Am. Chem. Soc. 131 (2009) 1376.[14] Program Fullprof.2k, version 3.30, Laboratoire Leon Brillouin, France, June

2005.[15] Z.D. Sha, X.M. Wu, L.J. Zhuge, Y.D. Meng, Physica E 35 (2006) 38.[16] T.Z. Ding, Q.G. Qi, J. Li, B.H. Ji, J. Rare Earths 20 (2002) 320.[17] Q. Wei, W. Cui, X. Long, Y. Xu, Chem. J. Chinese U 11 (1990) 1227.[18] S. Das Sarma, E.H. Hwang, A. Kaminski, Phys. Rev. B 67 (2003) 155201.[19] Y. Guo, W.X. Yuan, B. Song, Y.P. Xu, Powder Diffr. 23 (2008) 329.[20] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.[21] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988.[22] H. Li, B. Song, H.Q. Bao, G.Wang,W.J.Wang, Y.T. Song, X.L. Chen, J. Magn.Magn.

Mater. 321 (2009) 222.[23] H. Li, H.Q. Bao, B. Song, W.J. Wang, X.L. Chen, Solid State Commun. 148 (2008)

406.[24] H. Li, H.Q. Bao, B. Song, W.J. Wang, X.L. Chen, L.J. He, W.X. Yuan, Physica B 403

(2008) 4096.[25] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Nature (London) 430 (2004) 630.[26] J.D. Bryan, S.M. Heald, S.A. Chambers, D.R. Gamelin, J. Am. Chem. Soc. 126

(2004) 11640.[27] A.J. Behan, A. Mokhtari, H.J. Blythe, D. Score, X.H. Xu, J.R. Neal, A.M. Fox,

G.A. Gehring, Phys. Rev. Lett. 100 (2008) 047206.[28] B. Song, J.C. Han, J.K. Jian, H. Li, Y.C. Wang, H.Q. Bao, W.Y. Wang, H.B. Zuo,

X.H. Zhang, S.H. Meng, X.L. Chen, Phys. Rev. B 80 (2009) 153203.